Dendritic polymers, having a highly branched structure, are considered to exhibit physical and chemical properties and functions differing from those of conventional chain-form polymers. Dendritic polymers are generally classified into dendrimers and hyperbranched polymers. A dendrimer has a regularly controlled branching structure. Generally, a dendrimer has a radially and regularly branching structure in which polymer chains are branched from a core or a focal point serving as a center structure. The molecular weight of a dendrimer is virtually a single value; this is remarkably different from the case of conventional polymers, which exhibit a broad molecular weight distribution profile. Characteristic features of a dendrimer include low viscosity, high solubility, and amorphous nature, and these properties are of interest for various applications. In this connection, extensive studies have been carried out to impart new functions to the polymer through introduction of a variety of functional groups into end moieties serving as outer cores and a core serving as a center portion. In contrast, a hyperbranched polymer has structural regularity lower than that of a dendrimer and exhibits a wide distribution in terms of molecular weight and branching degree.
Among methods for synthesizing dendritic polymers, there have been known the “divergent method” in which branches are successively extended from a focal point or a core; the “convergent method” in which branching units for forming branch ends are successively connected and the thus-connected units are finally bonded to a focal point or a core; and self-condensation of a polyfunctional monomer of AB2 type (A and B are mutually reactive functional groups). Among them, self-condensation of a polyfunctional monomer of AB2 type is employed for synthesizing hyperbranched polymers exhibiting a molecular weight distribution, while the “divergent method” and “convergent method” are employed for synthesizing dendrimers having virtually a single molecular weight. The “divergent method” has some drawbacks. That is, the number of the outermost positions serving as reaction points increases as the generation number increases, thereby readily forming defects, and excessive amounts of reaction reagents are required so as to prevent defect formation. In contrast, the “convergent method” is advantageous. That is, the method is remarkably effective means for synthesizing, with high efficiency, a high-purity dendrimer having no defects, from the viewpoint of no requirement for excessive amounts of starting materials and easiness of purification of synthesis intermediates (see J. M. J. Frechet et al.; Chem. Rev. 101, 3819-3867 (2001)).
Examples of known repeating structures of the dendrimers synthesized through the aforementioned “convergent method” include polyaryl ether, polyaryl-alkylene, polyarylene, polyalkyl ether, polyarylalkene, polyamide, and polycarbonate. Specific examples include polybenzyl ether, polyphenylene, polyphenylene-vinylene, and polyphenylacetylene. Dendrimers will have a variety of functions in accordance with the combination of a core (center portion), end moieties (outer cores), and repeating structures for forming an inner skeleton. Thus, the repeating structure of dendrimers is a critical factor for determining characteristics of a functional material, and therefore, further new candidates of the structure and an effective synthesis method therefor are keenly demanded.
Meanwhile, thienylene moieties, having excellent electric properties and stability to heat and light, have been studied as base structures of conductive π-conjugated polymers and oligomers. In the field of dendritic polymers, there has been reported a dendrimer having an oligothiophene, structure serving as a core (center portion) and thienyl groups serving as end moieties (outer cores) (see J. M. J. Frechet et al.; J. Am. Chem. Soc., 120, 10990-10991 (1998) and J. Org. Chem., 63, 5675-5679 (1998)).
Among dendrimers having thienylene moieties in the repeating units (including synthesis methods therefor), a dendrimer having structural repeating units formed exclusively of a thenylene structure is disclosed (see Chuanjin Xla et al.; Organic Letters 2002, Vol. 4, No. 12, 2067-2070)).
The above synthesis method is a type of convergent method in which the generation of the dendrimer is increased through Grignard reaction or Stille coupling reaction. When Grignard reaction is employed, the reaction proceeds rapidly and exothermically, making temperature control in industrial production thereof difficult. In addition, in order to enhance yield of the synthesis, rigorous control of water content in the reaction system is essential. Thus, the Grignard-reaction-based method is unsuitable for large-scale production. When Stille coupling reaction is employed, a highly-toxic organotin compound must be used, and a severe deoxygenation step is required to enhance synthesis yield. Thus, the Stille coupling-based method is also unsuitable for large-scale production. In the aforementioned convergent method, building blocks serving as branch portions of repeating units are formed exclusively of 2,3-dibromothiophene, thereby limiting the dendritic structure. Except for the aforementioned dendrimers having thienylene skeleton structural repeating units and synthesis methods therefor, no other such dendrimers and synthesis methods therefor have been known.
Japanese Patent No. 3,074,277 discloses a hyperbranched polymer having thienylene-phenylene units serving as structural repeating units.
However, since the polymer is produced through polymerization based on Grignard reaction, highly regulated repeating structures such as those possessed by dendrimers cannot be provided. Therefore, the compounds synthesized through the Grignard-based method exhibit a broad molecular distribution profile, as conventional polymers exhibit, and functional groups are introduced at random into a core (center moiety) and end moieties (outer end portions), thereby failing to impart desired functions to the polymers.